Institute of Geophysics and Planetary Physics, University of
California, Los Angeles

The ionosphere of an unmagnetized planet, such as
Venus, is characterized by relatively high Pedersen conductivity in
comparison to the terrestrial ionosphere because of the weak
magnetic field. Collisional Joule dissipation of plasma waves
might therefore be an important source of heat within the Venus
ionosphere. However, any assessment of the importance of
collisional Joule dissipation must take into account the cooling
provided by electron heat conduction due to temperature gradients.
Once heat conduction is included we find that collisional Joule
dissipation is significant only in the bottomside ionosphere; waves
observed at or near the dayside ionopause, or at higher altitudes (>
150 km) within the nightside ionosphere do not cause significant
heating through collisional Joule dissipation. However,
lightning-generated
whistler mode waves propagate through the highly
collisional bottomside ionosphere, and we have performed detailed
wave propagation calculations where we self-consistently calculate
the heating due to Joule dissipation and the cooling due to heat
conduction. The heat conduction always exceeds the collisional
cooling from elastic collisions. Because the high collision
frequency at low-altitude results in a low thermal conductivity, a
steep temperature gradient is required to provide the heat flux.
However, this gradient thermally decouples the bottomside
ionosphere from higher altitudes. Collisional Joule dissipation of
lightning generated whistlers is not likely to have any
consequences for the global ionospheric energy budget. Cooling by
inelastic collisions, specifically the vibrational excitation of
CO,
further reduces the bottomside temperature. It is the inelastic
cooling rate that determines the atmospheric heating rate, any
excess heat again being carried away through heat conduction. We
find that for typical wave field amplitudes of 10 mV/m the
bottomside is heated to a few eV, while intense fields (100 mV/m)
result in bottomside temperatures of a few tens of eV. This high a
temperature may cause electronic excitation of the neutrals, which
could result in optical or ultraviolet emissions from the ionosphere
due to lightning. This possibility requires further investigation but
requires the incorporation of additional inelastic cooling processes,
such as electronic excitation of the neutral atmosphere.